In the art of earth drilling apparatus the efficient handling of the elongated sectional drill pipe members has been a problem of longstanding. In so-called rotary drilling the type of apparatus known in the art as a top drive has been developed to overcome certain problems associated with rotary drive devices which are mounted at the base of the drill rig, and otherwise known as rotary table drives. In top drive rigs the drill string rotating mechanism usually comprises an integral part of a head which is connected to the upper end of the drill string and is mounted for reciprocal traversal of a mast or drill tower to feed the drill string into the drill hole being formed and to hoist the drill string out of the drill hole. Rotary top drives eliminate the need for the special elongated drive member or kelly normally used with rotary table drive arrangements and thereby simplify the process of adding sectional drill pipe members with respect to the drill string. For example, when adding drill pipe to the drill string it is not necessary to first remove a kelly from the string each time a sectional pipe member is connected to the drill string, and the problem of a hole cave-in is avoided because the lower end of the drill string does not have to be raised a substantial distance off the bottom of the hole before a section of pipe is added. Moreover, in some earth formations it is often desirable to rotate the drill string while removing the same from the drill hole and such operation is not possible or at least becomes more complicated with rotary table type drive mechanisms.
The rotary table type of drive arrangement does offer certain advantages, namely, rotary table drive mechanisms are usually adapted to be used as a wrench for breaking out or disconnecting the joints between sectional pipe members. Furthermore, rotary table drives are more desirable for use on drilling rigs for drilling holes of substantial depth because greater rotational effort or torque is required and it is desirable to place the rotary drive mechanism on the base of the mast or on a lower portion thereof so as not to impose large twisting forces plus the weight of the rotary drive mechanism itself on the upper portions of the mast or tower.
Because of the abovementioned problems associated with conventional top drive and rotary table drive arrangements prior art drilling rigs have not been a versatile or as efficient as is desired, particularly rigs built for drilling holes within a wide range of hole depths.